The 15" depth is good.

I've never grown buttercrisp but if it's a leaf letuce, I harvest when I need a salad and it looks big enough to eat (smaller is usually better) if you're harvesting cut-and-come-again. Some types will also grow into a loose head (like green or red leaf lettuce from the store) and you can wait until it looks ready and cut the plant. With lettuce, if it gets too big or when the weather gets too warm and it bolts, it can get bitter. Sometimes you can soak the bitter out in cold water but not always. Just a heads-up :-)

If you have access to good sweet corn and limited garden space, I wouldn't plant corn. Some varieties give one ear/stalk/season and some give two. The two foot bed width could limit corn success as it's generally advised to plant at least 4 deep for good pollination. Corn is wind-pollinated and if pollination is poor, your ears will be missing kernels. Sometimes missing more than they have.

Lettuce can also be direct sown or if you prefer to transplant starts, it does well for me started on my covered front porch in a milk jug (wintersowing-style, even in spring or summer).

If you like peas and/or beans, they tend do do pretty well without much trouble. Squash, cukes & zucchini may be a good place to start too though it depends how bad the bugs are where you live.

Don't stress yourself out, gardening is sort of a learn-as-you-grow activity :-)

Here is a link that might be useful: Tales of a Transplanted Gardener

Thank you, that is very helpful information. I think I'll wait until next year on the corn then, when I can create a larger bed for it. I'm happy to have found a place with so much good information!